The endocrine system is vital to reproduction, controlling everything from the sex of an organism to the generation and release of gametes and even the sex drive. The endocrine system is responsible for the creation and release of hormones, the chemical messengers that regulate most of the body's systems. Disruptions to any one of several aspects of the endocrine system can shut down the reproductive system.
In humans and other mammals, testosterone, estrogen and progesterone are major sex hormones that control the release of gametes and, in females, also the preparation of the uterus for supporting young. After birth in mammals, hormones also control the release of milk for feeding young.
The endocrine system, as a whole, shares a role with the nervous system, handling long-term tasks for which the fast, energetic action of nerves is not needed. Hormones have no dedicated signal pathways like nerves do; instead, they travel from the glands that produce them through the blood to the sites where they are needed. The cells of the organs that hormones affect have special receptors for just the hormones intended to affect that organ or tissue. For instance, a change in sex hormones is received by cells in the ovaries, which prompts them to finish developing and release an egg.